1. Field of the Invention
The present invention relates generally to phased array antennas, and in particular to a system and method for tracking sources of radio frequency communication signals using phased array antennas.
2. Description of the Related Art
Phased array antennas are commonly used in both radar and communications applications. Phased array antennas comprise a plurality of elements coupled to a common source or load to produce a directive radiation pattern. The relative phases of the respective signals feeding the plurality of elements are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. Typically, such antennas produce a main lobe beam and two or more side lobes. Also, the sensitive axis of such antennas can be steered by controlling the phase between the plurality of elements using variable phase shifters.
The ability to electronically steer the beam of a phased array antenna provides many advantages (beam steering agility, speed, low observables, low profiles, and low maintenance) over its traditional, mechanically steered counterparts. But while the phased array antenna can be electronically steered to transmit or receive a beam in a particular direction, such antennas still require information regarding the direction in which the transmitting antenna must steer the beam in order to accurately direct the beam energy at the desired targets and to track such targets. This problem is especially important where the phased array antennas are used to transmit and receive communication signals between multiple objects at long distances or where low observables are important. Because such situations typically require relatively narrow antenna beamwidths and high agility, it is important to determine the location of the other object being communicated with as quickly and accurately as possible.
Current communications systems using phased array receive antennas use closed-loop tracking control techniques such as conical scan or sequential lobing to obtain the pointing information required to direct the associated transmit antenna beams in the desired direction. They also use open-loop tracking control techniques, with the pointing information provided by other systems such as navigation and inertial measurement systems.
When conical scanning a phased array communication antenna, the beam of the receive antenna is slightly offset from the antenna's nominal direction, then circularly rotated, thus producing in a shape akin to a cone between the antenna and the target. Ideally, a target transmitter located at the center of a conical scan will generate a constant return at all points in the conical scan, because the target transmitter will be offset from boresight by the same amount during the entire scan. However, when the target transmitter is not centered, the off-axis gain characteristic of the main lobe of the antenna will cause the return to vary in intensity or disappear altogether for short periods during the scan. The strength of the return is indicative of how close the center of the scan is to the actual target location, and this information can be used to direct the transmit antenna to the proper orientation. Sequential lobing is conceptually similar to conical scanning, except that measurements are taken at discrete positions rather than a continuous scan.
While conical scan or sequential lobing techniques are effective, they have a relatively slow response time. In particular, the conical scan controller often cannot operate fast enough to ensure adequate tracking when the communications platform is rolling higher rates, including those in the order of 10 degrees per second or more.
What is needed is a system and method for using a phased array receive antenna to concurrently receive a signal transmitted by a remote transmitter, to determine the angular location of the remote transmitter using the transmitted signal, and to use that angular location to direct a transmit antenna to transmit information in the direction of the transmitter. The present invention satisfies these needs.